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Comparing acidity of 2 substituted benzoic acids

  1. Jun 11, 2017 #1
    Ok so i am a beginner in organic chemistry and i came across comparing acidity.
    Now according to various books i referred,anthranilic acid/ortho-amino benzoic acid is less acidic than benzoic acid.There was not sufficient explanation and therefore,my doubt.
    According to my book,in o-hydroxy benzoic acid,the conjugate base is stabilized by hydrogen bonding between OH and COO negative ion,thus making the compound more acidic.
    When i apply the same logic to this argument,apparently it fails.I want to know why.
    Note-I am aware about nh2 being an exception to the ortho effect due to its small size.But Then by same logic,shouldnt OH be an exception too
     
  2. jcsd
  3. Jun 12, 2017 #2

    DrDu

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    Note that the amine is a much better base than phenolic OH. Hence it may stabilize the acid function.
     
  4. Jun 12, 2017 #3
    Thank for replying but can you please elaborate a little bit more.
     
  5. Jun 12, 2017 #4
    I think he's talking about zwitter ion.
     
  6. Jun 12, 2017 #5

    DrDu

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    ... Or at least an intramolecular hydrogen bridge.
     
  7. Jun 12, 2017 #6
    EDIT: I now understand why you were talking about this. The title of this thread is quite misleading. The OP actually asks in the OP the difference in the chemistry behind why the acidity change between anthranilic acid and benzoic acid is different from acidity change between salicylic acid and benzoic acid.

    In shorter words, he is asking why acidity decreases from benzoic acid to anthranilic acid, and NOT why anthranilic acid and salicylic acid have different acidity.


    The answer is most likely zwitterion.
     
    Last edited: Jun 12, 2017
  8. Jun 13, 2017 #7

    DrDu

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    upload_2017-6-13_8-50-1.png
    Something like this. Let's leave it open, to what percentage this is zwitterionic. Acylic amino groups are not as basic as aliphatic ones, so there will be some equilibrium. However, it is hard to say which side is really dominant.
     
  9. Jun 13, 2017 #8
    Pardon me for my limited knowledge in organic chemistry,and therefore to know that i have grasped the concept i will explain what i understood and if i am wrong correct me please.If i am correct,acknowledge the same please.
    So in Anthranilic acid there are adjacent base and acid;Therefore,there is a a possibility of transfer of H+ from COOH to NH2,forming a zwitter ion.
    Now although COOH loses a proton,the molecule doesn't and therefore the acidity gets reduced as the probability of molecule losing its proton gets reduced.The extent of this reduction depends on the probability of zwitter ion formation,but it is safe to say that in comparison to simple benzoic acid,it(anthranilic acid ) is less acidic.
    I thank you once more,It was really very helpful.
     
  10. Jun 13, 2017 #9
    Hmmm...I am not sure if this argument provides a good insight.
    anthraniline acid.jpg

    These hydrogen bond will stabilize both the ionized state and not ionized state.

    Your argument explains how hydrogen is prevented from leaving the molecule, and I believe salicylic acid to have the stronger intramolecular hydrogen bond than anthranilic acid. That means salicylic acid should show even weaker acidity than anthranilic acid, compared to benzoic acid from this perspective. In the ionized state, the salicylic acid would still show stronger hydrogen bond than anthranilic acid. That means salicylic acid should show even stronger acidity than anthranilic acid, compared to benzoic acid. In summary:
    Untitled.jpg
    Since we already know the result, the above argument says that in the case of anthranilic acid, the ionized state stabilization effect is weaker than non-ionized state stabilization effect, while the opposite applies for salicylic acid. I don't think this can provide any good insight into why anthranilic acid show weaker acidity than benzoic acid while salicylic acid show stronger acidity than benzoic acid. Due to the qualitative nature of the explanation, we cannot judge which effect dominates over the other.

    The explanation of the zwitterion, in comparison, can clearly explain OP's question.
     
  11. Jun 13, 2017 #10

    Ygggdrasil

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    From Wikipedia, here are the pKa values for each of the carboxylate groups:
    benzoic acid pKa = 4.202
    2-aminobenzoic acid pKa = 2.14
    2-hydroxybenzoic acid pKa = 2.97

    So, 2-aminobenzoic acid is more acidic than benzoic acid, and it is also more acidic than 2-hydroxybenozic acid.

    Therefore, it is likely that intramolecular hydrogen bonding stabilizes the conjugate base of both 2-aminobenzoic acid and 2-hydroxybenzoic acid. 2-aminohydroxybenozic acid is more acid than 2-hydroxybenzoic acid because of zwitterionic effects (i.e. the positively charged ammonium group will help to stabilize the neighboring negatively charged carboxylate in the conjugate base).
     
  12. Jun 14, 2017 #11

    DrDu

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    Yes, but the amine being much more basic than the hydroxide, it will be a better hydrogen bond acceptor and worse hydrogen bond donator. I.e., comparing anthranilic and salicylic acid, in the former the unionized state will be relatively more stabilized than the ionized state resulting in a lower acidity.

    Btw, the Zwitterionic structure of anthranilic acid has higher energy than the nonionic form of anthranilic acid, i.e, it can't be used to explain the higher pKa:
    http://www.mdpi.com/1422-0067/15/11/19562/htm
     
  13. Jun 14, 2017 #12
    I think wikis pka values are wrong.My book says it is weaker than Benzoic
     
  14. Jun 14, 2017 #13
    And moreover salylic acid has stronger H bonding-correct But
    According to my book it says that HBonding is possible in both ionised and unionised state,But as we know O negative forms stronger H bond than simple O ie it stabilises conjugate base more than undissociated acid and therefore probability of H bonding is more in conjugate base rather than undissociated state,therefore salicylic acid is stronger but as Nh3 forms zwitter ion instead,the same argument doesn't apply.
    I think I have got it.
    I Use Peter Sykes as my reference book
     
  15. Jun 14, 2017 #14

    DrDu

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    As I mentioned above, literature excludes the formation of zwitter ions in anthranilic acid.
    Ygggdrasil showed that both anthranilic acid and salicylic acid are stronger than benzoic acid, so you are right that the anions are more strongly stabilized than the neutral acids in both anthranilic and salicylic acid whence both have a lower pKa than benzoic acid. However, comparing salicylic and anthranilic acid, the neutral acid is more strongly stabilized in the case of anthranilic acid, whence it is a weaker acid than salicylic acid.
     
  16. Jun 14, 2017 #15
    ok so to conclude,Relative order of acidity is Salicyclic acid>anthranilic acid>benzoic acid.
    Thank you guys for your inputs.
     
  17. Jun 14, 2017 #16

    DrDu

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  18. Jun 14, 2017 #17

    Ygggdrasil

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    In order from most acidic to least acidic: 2-aminobenzoic acid, 2-hydroxybenzoic acid, benzoic acid.

    Here are the pKa values for the carboxylic acids from the NIH database:
    Benzoic acid: 4.204 https://pubchem.ncbi.nlm.nih.gov/compound/243#section=Dissociation-Constants
    2-hydroxybenzoic acid: 2.98 https://pubchem.ncbi.nlm.nih.gov/compound/338#section=Dissociation-Constants
    2-aminobenzoic acid: 2.14 https://pubchem.ncbi.nlm.nih.gov/compound/227#section=Dissociation-Constants
     
  19. Jun 14, 2017 #18
    Why god why? LOL
    Somebody explain now why salicyclic is weaker
     
  20. Jun 16, 2017 #19
    So many different sources saying so many different pKas for the molecules, and every time I have to change my interpretation. lol. At this point, I almost feel like doing the experiments myself to see for myself. I do not have a free pH meter available right now, so I can't do that.

    Can someone confirm the sources? The sources provided by Ygggdrasil seems to be from different sources. Also, the temperature is different (and is not provided for anthranilic acid).
     
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